MRI is a type of imaging technology involving biomagnetics and nuclear spin. MRI has developed rapidly together with computer technology, electronic circuit technology, and superconductor technology. The main principles of the phenomenon of magnetic resonance are as follows. If an atomic nucleus contains a single proton, as is the case, for example, with the nuclei of the hydrogen atoms that are present throughout the human body, this proton exhibits spin motion and resembles a small magnet. Moreover, the spin axes of these small magnets lack a definite pattern, and if an external magnetic field is applied, the small magnets will be rearranged according to the magnetic force lines of the external field. Specifically, the small magnets will line up in two directions, either parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is called the negative longitudinal axis; the atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field such that their spin axes deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance. This is the phenomenon of magnetic resonance. Once the spin axes of the excited atomic nuclei have deviated from the positive or negative longitudinal axis, the atomic nuclei have a transverse magnetization component. Once emission of the RF pulse has stopped, the excited atomic nuclei emit an echo signal, gradually releasing the absorbed energy in the form of electromagnetic waves, such that the phase and energy level of the nuclei both return to the pre-excitation state. An image may be reconstructed by subjecting the echo signal emitted by atomic nuclei to further processing, such as spatial encoding.
In an MRI system, the gradient coils are key components for producing a linear gradient magnetic field. The gradient coil at least includes a main coil set and a shield coil set; the main coil set and shield coil set each further include three layers of coils. Copper wire may be used as a conductor to weld together the main coils and shield coils using a flux. In order to save costs while reducing the weight of the gradient coils, it is possible to develop and use aluminum wire, but due to its tendency to oxidize easily, aluminum wire is difficult to weld using flux. Thus, it is not possible to manufacture aluminum wire gradient coils on a large scale.
U.S. Pat. No. 8,067,939 proposes the use of two conductors of different shapes to form a gradient coil, wherein planar conductors (e.g., made of aluminum material and formed by waterjet cutting or laser cutting, etc.) are welded to ordinary copper conductors with a rectangular cross section. This solution does not suggest how to solve the problem of oxidation that arises when copper and aluminum are welded together using flux.